1. Field of Invention
This invention relates to finishing powders, particularly, but not exclusively, to finishing powders for condoms.
2. Description of Related Art
It is well known to use finishing powders in the man and production of both natal and synthetic rubber articles such as gloves and condoms. In the case of condoms, the finishing powder fulfills a number of functions, including the reduction of tack inherent on the untreated natural rubber surface and the provision of a low friction surface in order to facilitate transport of the condom on to mandrels and such like during automated processing. The finishing powder also enables a low friction surface to be maintained when the condom is in a rolled state to allow the condom to be handled easily by a foiling machine during packaging. Finally, in the case of condoms, the finishing powder allows the condom to be unrolled easily by the user.
A number of different finishing powders have been used for condoms in the past, including talc, mica, lycopodium and cross-linked maize starch. Owing to adverse clinical reports about mica, talc and lycopodium one of the most commonly used finishing powders now is cross-linked maize starch. This has many advantages as a finishing powder since it is available in a high state of purity, it is well tolerated by the body, it is bio-absorbable and it confers a low friction surface. However, more recently cross-linked maize starch has been shown to have a number of problems. For example, its bio-absorbable nature means that when used in aqueous slurries it is prone to microbial attack. To deal with this it is generally necessary to add biocides to the aqueous formulation, and these present a toxicity risk. There have also been reports that proteins present in, for example, natural rubber medical gloves can bind to cross-linked maize starch and thus possibly give an airborne route of exposure to the proteins, which are known to cause sensitisation problems in some users. (Refer, for example, to Beezhold, D. H. and Beck W. C. Archives of Surgery (1992), 127, 1354-1357.)
As a consequence of the above disadvantages, it has been recognised that there is a need to move away from starch to more advantageous finishing powders. One alternative has been to use inorganic powders such as calcium and magnesium carbonate. These compounds present advantages over cross-linked maize starch in that they have a good safety record, they are generally not susceptible to microbial spoilage and they do not bind the proteins found in natural rubber latex. On the whole, therefore, the industry has found it acceptable to use such compounds as satisfactory alternatives to starch.
However, we have found certain problems with inorganic carbonate finishing powders, which problems have hitherto not been recognised or appreciated. One problem is that the carbonates used up till now have been produced by standard manufacturing processes and these carbonates have an irregular or spicula particle morphology. As a consequence, they do not reduce friction anything like as well as starch. In particular, we have found that these standard carbonates, particularly magnesium carbonate, can give rise to problems during rolling and unrolling of a condom. Typically, when a condom is rolled off a shaped mandrel to give the familiar rolled condom, the rolling is applied from one side only. The action of rolling applies a localized strain to the side of the condom contacting the rolling belt or wheel, and if this strain is not dissipated within the condom, then one side of the condom can be more tightly rolled than the other. In addition, any misalignment (perhaps of only a few thousandths of an inch) can exacerbate this problem. Such a discrepancy of strain within the condom can give rise to difficulties when the user tries to unroll the condom. Cross-linked maize starch is very efficient at releasing these strains because of the low degree of friction it confers to the surfaces of the condom both during and after rolling. By contrast, however, we have found that when standard magnesium carbonate is used any differential stains created by the rolling are not released, but are retained within the rolled structure, giving rise to complaints of difficult unrolling by the condom user. Another problem we have discovered in using standard magnesium carbonate as a condom finishing powder is that the powder can impede migration of condom lubricant within the rolled condom structure. Typically lubricant is applied to a rolled condom just before packaging and it migrates along the rolled condom film whilst inside the foil pack. The inhibition of migration of lubricant appears to arise because of affinity between standard magnesium carbonate and the lubricant typically used for condoms. The lubricant appears to become “bound” to the standard magnesium carbonate, forming a stiff paste which effectively seals off the remainder of the condom body, preventing the rest of the lubricant from migrating down the condom body. We have found that 1 g of standard magnesium carbonate can bind up to 4.4 g of lubricant. The resulting absence of lubricant towards the closed end of the condom can again add to the difficulty in unrolling the condom, and generate user complaints about “dry” condoms.